The present invention relates to digital cable systems and more specifically to a digital cable system having an improved statistical remultiplexer adapted to deliver video on demand with low latency in response to a viewer command.
The increasing popularity of video on demand (“VOD”) delivery of cable programming is forcing cable operators to dedicate more bandwidth to meet subscriber demand. This demand poses problems especially for cable operators whose cable systems are operating at a level that is close to the available bandwidth.
Originally, the available bandwidth of a cable system was divided into a limited number of analog channels each having a 6 Mhz bandwidth. Because digital channels take up less bandwidth than the old analog channel, it is now possible to multiplex several digital streams in the bandwidth originally assigned to a single analog channel. In most digital cable systems, the 6 Mhz bandwidth can now carry a multiplexed stream of video data at a rate of about 39 Mb/s or about ten to 12 digital streams to the subscribers' set top box. Digital cable systems have enabled cable providers to offer more channels and other services, such as VOD.
Digital cable systems employ the MPEG2 encoding standard for storing and transmitting audio and video in compressed format in order to transmit the streams in the available bandwidth. Further, most cable systems employ variable bit rate encoding where the bit rate allocated to each stream varies depending on the amount of complexity, such as frame to frame motion or picture details, in the video stream. Variable bit rate encoding means that at different times, different numbers of bits are allocated to the same stream.
In order to multiplex as many channels as possible into a single video stream, cable operators often rely on a technique called statistical multiplexing to ensure that the available bandwidth is efficiently utilized. The statistical multiplexing technique operates on the premise that not all channels will require high bit rates at the same time. Thus, when one channel temporarily needs a high bit rate, some bandwidth is taken from the other channels so that the total bandwidth limit of about 39 Mb/s is not exceeded at any time. With this technique, each digital television channel gets a constantly varying share of the total available bandwidth. In essence, statistical multiplexing is a technique that combines the bit stream representations of several channels into a single bit stream representation, or aggregated output transport stream, that complies with the constraint imposed by the bit rate limitation. Statistical multiplexing is frequently used to pack more broadcast channels onto a single 6 MHz QAM channel. Unfortunately the latency introduced by statistical multiplexing makes it less appealing for VOD applications.
A statistical multiplexer (“stat mux”) is a common piece of head-end equipment deployed by cable systems to efficiently utilize the available bandwidth of the cable system. The stat mux does not pre-define the allocation of bandwidth to a given channel. Rather, bandwidth allocation, in prior art devices, is calculated based on the instantaneous needs of each bit stream. Unfortunately, the bandwidth allocation process can result in significant latency because the stat mux must “wait” to look into the future to determine bandwidth requirements for each channel in order to perform intelligent allocation of bandwidth.
The stat mux solves the problem of matching the MPEG2 encoded video streams with the available bandwidth by using intelligent buffering to smooth out peaks of the individual video streams within the aggregated output transport stream. Many stat muxes perform the intelligent buffering by incorporating a look-ahead buffer to dynamically change bandwidth allocation in response to demands from each stream. The look-ahead buffer improves performance because the stat mux has statistical information about the future bandwidth requirements of each stream. Look-ahead buffer schemes work better if the buffer length is increased, but unfortunately increasing the buffer length means that the stat mux must include more memory, will require more processor time and will result in longer delay when re-multiplexing the streams into an aggregated output transport.
Longer buffers have been accepted for broadcast applications because such buffers enable cable operators to multiplex more channels into a single stream. However, long look-ahead buffers introduce a significant problem when cable operators offer VOD programming because the latency caused by the need to empty and re-fill the buffer when, for example, a viewer selects a VOD function request such as pause, rewind or scan forward. There will be considerable delay in responding to VOD function requests, such as pause, rewind or scan forward. This delay, which can range from one to two seconds or more, will be noticeable and annoying to the viewer. Further, if there is a one- or two-second lag before the viewer's function request is recognized, most viewers will be unwilling to utilize VOD and the cable operators will suffer reduced revenues. By limiting the length of the look-ahead buffer, the delay can be decreased but video quality will degrade and the number of channels a cable operator can carry will be limited. Thus, there is great need to minimize the delay that the viewer perceives when watching VOD without degrading the video quality or limiting the number of channels made available to the viewer. What is needed is a stat mux that maintains video quality and channel capacity while minimizing latency when a viewer selects a VOD function. Indeed, there is a great need to make the viewing experience more like watching a movie or other video content recorded on a DVD or a tape cassette.